1. Field of the Invention
The present invention relates to a surface acoustic wave device used for a resonator, band filter, or other suitable device, and more particularly, to a surface acoustic wave device wherein an electrode film of Au is formed on a LiNbO3 substrate, using second leaky surface acoustic waves.
2. Description of the Related Art
Surface acoustic wave devices are widely used in band filters for portable mobile communication devices, due to the high performance, light weight, and small size, thereof.
With surface acoustic wave filters, the operation frequency F is determined by the ratio of the phase speed V of the surface acoustic wave relative to the finger period L of the interdigital transducer (IDT). That is to say, the operation frequency F is represented by the relation F=V/L.
In recent years, demand for surface acoustic wave filters with high operation frequencies is increasing, and accordingly, development of surface acoustic wave filters using surface acoustic waves with high phase speed V is being undertaken.
The band width of the surface acoustic wave filters is dependent upon the electromechanical coupling coefficient ks2 of the surface acoustic wave. That is to say, in the event that the electromechanical coupling coefficient is great, a surface acoustic wave filter with a wide band width is obtained, and on the other hand, in the event that the electromechanical coupling coefficient is small, a surface acoustic wave filter with a narrow band width is obtained. Accordingly, there is the need to determine the electromechanical coupling coefficient ks2 according to the usage.
On the other hand, loss occurring in propagation of the surface acoustic wave, i.e., the propagation loss, causes deterioration of the insertion loss of the surface acoustic wave device, or causes deterioration of the impedance ratio which is the ratio of the resonance resistance of the surface acoustic wave resonator or the impedance at the antiresonant frequency point thereof, as to the impedance at the resonant frequency point thereof. Accordingly, a small propagation loss is preferable.
Furthermore, the change in the operation frequency dependent upon the temperature is preferably small with regard to the surface acoustic wave device. In the event that the change is great, the practically-available pass band width and stop band width are reduced, leading to abnormal oscillation in the event of a surface acoustic wave resonator making up an oscillation circuit. Accordingly, a surface acoustic wave filter wherein the change in operation frequency per 1° C. (TFC) is as small as possible is desired.
As a surface acoustic wave used for the surface acoustic wave devices, the Rayleigh wave and leaky surface acoustic wave are known. In many cases, the leaky surface acoustic wave propagates at a phase speed higher than with the Rayleigh wave. Accordingly, with the surface acoustic wave devices used for the high-frequency band, in many cases, a leaky surface acoustic wave propagating on the substrate such as any of 36°-Y-cut-X-transmission LiTaO3 substrate through 42°-Y-cut-X-transmission LiTaO3 substrate, 41°-Y-cut-X-transmission LiNbO3 substrate, or 64°-Y-cut-X-transmission LiNbO3 substrate is used, wherein the principal component is the transverse wave component (u2 component) horizontal to the surface acoustic wave propagation direction. The aforementioned leaky surface acoustic wave propagates at a phase speed between 4000 m/s and 4500 m/s.
In recent years, the second leaky surface acoustic wave with the phase speed between 5000 m/s to 7000 m/s, higher than with the conventionally-used leaky surface acoustic waves, attracts attention, wherein the principal component is the longitudinal wave component (u1 component). The surface acoustic wave devices using the aforementioned second leaky surface acoustic wave are disclosed in the non-patent documents 1 through 3, and the patent document 1 described below.
A surface acoustic wave device using leaky surface acoustic wave with the phase speed of 6656 m/s on a lithium tetra-borate substrate, generally formed of a longitudinal wave component (u1 component), is disclosed in the non-patent document 1, “Longitudinal Leaky Surface Acoustic Wave On Li2B4O7” (Proceedings of the 1994 Spring IEICE General Conference, A443, 1994).
Furthermore, a surface acoustic wave device using the second leaky surface acoustic wave, generally formed of the longitudinal wave component, which propagates on a LiTaO3 substrate or a LiNbO3 substrate, is disclosed in the non-patent document 2, “Characteristics of Leaky Surface Acoustic Waves Propagating On LiNbO3 And LiTaO3 Substrates” (S. Tonami, A. Nishikata, Y. Shimizu, Jpn. J. Appl. Phys. Vol. 34 (1995) pp. 2664-2667). The second document describes that the surface acoustic wave device using a LiNbO3 substrate exhibits the maximum electromechanical coupling coefficient k2 of 12.9%, the propagation loss in the electrically open-circuit state (free surface), and in the electrically short-circuit state (short-circuit surface), of approximately 0.06 dB/λ and 3.9 dB/λ, respectively, with the Euler angles of (90°, 90°, 36°), wherein the propagation loss on the short-circuit surface is greater than as compared with the free surface. Furthermore, the second document describes that the surface acoustic wave device exhibits the frequency-temperature property TFC on the free surface and the short-circuit surface of approximately 67 ppm and 78 ppm, respectively, with the Euler angles of (90°, 90°, 36°).
Furthermore, a surface acoustic wave device is described in the non-patent document 3, “Characteristics of the Second Leaky Surface Acoustic Waves with Low Propagation Loss on LiNbO3” (Tonami, Nishikata, Shimizu, Proceedings of the 1996 IEICE General Conference, A305, 1996), wherein the second leaky surface acoustic wave propagates with markedly small propagation loss of 0.00362 on a LiNbO3 substrate with the Euler angles of (82°, 92°, 37°). Now, making a comparison between the propagation loss with the Euler angles of (90°, 90°, 37°) described in Table 1 in the non-patent document 3 and the propagation loss indicated in FIG. 6 in the aforementioned non-patent document 2, the aforementioned propagation loss is considered to be caused on the free surface.
Furthermore, the patent document 1 (Japanese Unexamined Patent Application Publication No. 8-288788) describes that in the event that the longitudinal-wave pseudo-surface acoustic wave is adjusted so as to propagate with a phase speed less than the “fast transverse wave” and the “slow transverse wave”, the longitudinal surface acoustic wave propagates with the propagation loss of zero, and in the event that the surface acoustic wave device is formed of a LiNbO3 substrate with an electroconductive film made of Au with the thickness of KH of 0.3 (H/λ=4.8%) or more, the longitudinal surface acoustic wave propagates with the propagation loss of zero, as well.
On the other hand, a non-patent document 4, “SiO2LiTaO3, LiNbO3 Structure Acoustic Wave Materials Fabricated by Plasma CVD Method” (Nakajyou, Yamanouchi, Shibayama, IEICE Ultrasonic Group Technical Report US80-3, 1980), describes a surface acoustic wave device with a SiO2 film formed thereon exhibits improved TCF with regard to the leaky surface acoustic wave.
The surface acoustic wave device using the longitudinal leaky wave propagating on a lithium tetra-borate substrate described in the non-patent document 1 exhibits a small electromechanical coupling coefficient of 2.7%. Accordingly, a surface acoustic wave filter having such a configuration is formed with a narrow band width, and accordingly, cannot exhibit sufficient properties required for RF filters used for cellular phones or other devices.
With the surface acoustic wave device using the second leaky surface acoustic wave propagating on a LiTaO3 substrate or a LiNbO3 substrate described in the non-patent documents 2 and 3, a suitable electromechanical coupling coefficient can be obtained, and also the propagation loss is small on the free surface. However, the surface acoustic wave device has the disadvantage of great propagation loss in the event that the substrate surface is electrically short-circuited by forming a metal film thereon. The propagation loss of the IDT is similar to that in a case of the short-circuit surface, and accordingly, a surface acoustic wave device with a small propagation loss on a short-circuit surface or a metallized surface is expected. However, the surface acoustic wave device using the second leaky surface acoustic wave described in the non-patent document 2 or 3 cannot exhibit required properties.
On the other hand, with the configuration disclosed in the patent document 1, although the surface acoustic wave device exhibits the propagation loss of zero, the surface acoustic wave propagates generally at the same phase speed as the Rayleigh wave, which is only around 3000 m/s. Accordingly, the surface acoustic wave device has difficulty in handling high-frequency signals. In addition, the surface acoustic wave device exhibits TFC of only around 79 ppm. Accordingly, in the event that the surface acoustic wave device is used at a temperature between around −20 and +80° C., a great change in frequency of 7,000 ppm is caused. Accordingly, the surface acoustic wave device has difficulty in exhibiting acute filter properties all over the operating temperature range.
Note that while the above-described non-patent document 4 describes a method for improving the temperature property TFC of the surface acoustic wave device using the leaky surface acoustic wave by forming a SiO2 film thereon, conventionally, no configuration for improving the temperature properties of the substrate using the aforementioned second leaky acoustic wave has been known.